Nobody tracks individual New Yorkers across decades, so every turnover figure here is an estimate built from official totals. The yearly ledger of births, deaths and moves in section II is enough to estimate roughly how much of the city changed hands between any two years: take the people here at the start, apply each year's exit rate (deaths plus out-moves) to them, and count what remains against everyone born or arrived since. The further back a window reaches and the longer it runs, the rougher the estimate.